Easy, people attend to go off-topic REALLY fast and that always leads to big arguements and flame which would result into ban/mutes.
By locking the thread, they make sure that situation won't happen anymore so the issue won't get bigger.
Example ;
https://freestylersworld.com/showthre...git-kids/page1 this thread should be closed a.s.a.p in my opinion because it's totally off topic even tho it's in the rage/ego section.
I bet some one in that thread will be reported and that would have consequences to that person.